Morphological and geochemical properties of soil accumulated in hedge-induced terraces in the Massif Central, France.

摘要

Hedges are part of the landscape in many regions of the world. Among many important roles, they limit soil translocation. Hedges perpendicular to the slope at the lower end of sloping fields result in the formation of soil terraces. Quantification of fluxes of matter at the landscape scale has shown that terraces cannot be neglected. In this study, we try to quantify and explain the origin of the morphological and geochemical properties of terraces. The morphology of the terraces and corresponding stocks of soil material and chemical elements are assessed through a microtopographic study. The 11-ha study area is located on a rolling landscape in the Massif Central (France). The study is focused on three particular terraces. Two DEMs (2.5-m resolution) were established on the study area. The first DEM (DEM1) represents the actual elevation, using 4600 elevation spots. Elevation cross-sections were computed to determine the extent of the terraces, and a second DEM (DEM2) was then calculated, excluding all the elevation spots located in terraced areas. The thickness of soil material stored in each terrace is given by DEMst=DEM1-DEM2. It varies between 0 and 0.63 m, representing a soil accumulation of 3-7 m3 m-1 hedge length. One-hundred and seventy-three samples were taken in the topsoil, and the content in some major and trace elements (Ca, Mg, K, Fe, Mn, Cr and Co) measured and mapped using ordinary kriging. The stock of these elements accumulated in the terraces was computed and compared to the stock eroded considering uniform erosion from the upper part of the fields. Results show a difference in stocks exceeding more than 20% for several elements, showing that uniform erosion is not a satisfactory explanation for the accumulations observed in the terraces. A higher contribution of the area located immediately upslope form the terraces results in a better agreement in the stocks comparison for most of the chemical elements studied. Evidence from coarse fragments study, particle size distribution, soil depth in the upslope part of the fields and microtopography show that the formation of the terraces is probably mainly due to redistributions through tillage. The geochemical properties of the terraces are probably exclusively the result of this mechanical redistribution, except for Mn and Co. Indeed, it is likely that since the plantation of the hedges, seasonal waterlogging conditions have significantly affected the mobility of these two elements through geochemical processes that resulted in their leaching downwards as well as perhaps out of the field..